Metal preheat-treat coating



United States Patent METAL PREmAT-TREAT CHEATENG Edward B. Schneider,Canoga larlr, Calif., assignor to North American Aviation, Inc.

No Drawing. Filed Mar. 39, 1961, Ser. No. 99,379 5 Claims. (Cl. 148-22)This application is a continuation-in-part of my application Serial No.823,313, filed June 29, 1959, now abandoned for Metal Preheat-TreatCoating.

This invention relates to a composition and method for preventing oxidescale formation during the heat treatment of steels particularlystainless steels, and various other metal alloys. It is morespecifically concerned with a composition which can be easily applied tometal surfaces to protect them from oxidation and other degenerativeprocesses during heat treatment and which, depending upon the metal tobe protected, either spalls upon cooling or is readily removable byimmersion in standard solutions for substantially reduced periods oftime to leave a bright metal finish.

The heat treatment of various metals and principally steels, copper,copper alloys, and those now generally referred to as superalloys, giverise to oxidation and alloy reactions which markedly deteriorate thesurfaces of the heated articles. These reactions not only cause thedestruction of a portion of the surface metal but also result in theformation of a heavy layer of metal oxide scale. In most cases thisscale can only be removed by a costly and time-consuming sand blastingoperation. Where conventional protective coatings are used and scaleformation is not, therefore, as aggressive as is normal, the oxide scalemust be eliminated by immersion and retention in acid pickling baths forextended periods of time.

When carbon steels are subjected to heat treatment excessive oxidationof the surface is normally encountered and may greatly reduce theoverall dimensions of the piece being heat treated. When this occurs, itis necessary to remove the oxide scale layer which forms by machining orby some other means in order to obtain a surface having the physicalcharacteristics of the parent metal.

Where scale forms, or other surface modifying reactions occur uponheating, on article cannot be heat treated in its final form.Furthermore, since machining often alters the mechanical properties ofthe metal, it will be impossible, in many instances, to obtain the exactsurface condition desired unless it is possible to heat-treat an articlein its final form.

Although oxide scale preheat-treat inhibitors are known to the art, noneof the conventional compositions function in a scale preventativecapacity insofar as steels, and especially the precipitation hardenableand corrosion resistant steels, are concerned. The most that isaccomplished by such compositions is a reduction and slight modificationin the character of the scale which is formed on heat treatment. A sandblasting or rather severe acid pickling of the treated article is stillrequired. It is also well known that the scale which normally forms inthe heat treatment of superalloys has heretofore been impossible toremove except by abrasive means. These alloys are generally comprised ofnickel, chromium and cobalt as major constituents and are discussed inthe ASM, Metals Handbook (1948), on pages 566-569.

It is, therefore, a principal object of this invention to provide aprotective coating composition and method which will eliminate theforegoing deficiencies of known heat treat techniques and coatingcompositions. A further object is to provide a composition and methodfor protecting the surface of metal articles during their heattreatment. Still another object of this invention is to provide a metalarticle having a coating which protects the surface during heattreatment of the metal. Another 3,i?8,322 Patented Apr. 13, 1965 objectof this invention is to provide a composition and method whichcompletely prevent the formation of an oxide scale and eliminatesundesirable changes such as carburization and other surface reactionsoccurring during the heat treatment of stainless steels. A furtherobject is to furnish a composition and method which inhibit theformation of scale as the result of heat treatment of superalloys and toso modify any scale which does form to permit its ready removal by acidpickling. Still 'another object is to provide a pre-heat-treat coatingcomposition which not only protects the surface of the stainless steelsagainst carburization and the formation of oxide scale, but also spallsreadily from the surface of the coated article during cooling to exposea brightsurfaced, heat-treated article. An additional object of theinvention is to provide a protective coating composition which permitsthe heat treatment of metal articles in their final shape and form.

These as well as other objects will become apparent from the followingdetailed description of the invention.

The objects of this invention are in part accomplished by providing aglass-ceramic heat treatment protective coating composition for metalsurfaces, which composition comprises a non-metallic aluminumconstituent, a silicon oxide constituent, a non-metallic leadconstituent, and a non-metallic boron constituent. The composition maybe prepared by blending oxides of aluminum, silicon, lead, and boron or,alternatively by mixing compounds containing one or more of thealuminum, silicon, lead, and boron constituents. The oxygenatedcompounds of the constituents are preferred ingredients of thecomposition. For example, a coating composition of this invention may beprepared by blending 40 parts of alumina with from 60 to parts ofsilica, -130 parts of litharge, and from 40 to 50 parts of boricanhydride. Alternatively, compositions of this invention may be preparedby blending together appropriate quantities of oxygenated compounds,ceramic frits and glass compositions containing oxygenated compounds ofaluminum, silicon, lead, and boron. For example, a composition of thisinvention may be prepared by blending parts of kaolin containingapproximately 39 percent by weight aluminum oxide, 45 percent by weightsilicon dioxide, and 16 percent water with about 300 paits by weight ofa ceramic frit prepared from a lead borosilicate glass and whichcontains approximately 60 to 65 percent lead oxide, 10 to 15 percentsilicon dioxide, and 20 to 25 percent boric anhydride.

The protective coating of this invention ordinarily contains from l040percent aluminum constituent, based on aluminum oxide, from 10-40percent silicon constituent, based on silicon dioxide, from 20-70percent lead constituent, based on plumbous oxide, and 5-25 percentboron constituent based on boric anhydr-ide. In a preferred embodimentof the invention, the proportion of boron constituent as boric anhydrideis from 14-18 percent and the amount of the lead constituent, as leadoxide, is from 40 to 50 percent. These relative percentage figures arewith respect to the total aluminum, silicon, lead, and boron content ofthe composition calculated as the respective oxides. When otherconstituents are present in the composition they do not affect therelative ratios of aluminum, silicon, lead, and boron constituentspresent.

Other compositions of this invention are prepared with the inclusion ofa halide flux such as calcium chloride or barium chloride, a binder anda diluent.

The compositions of this invention, when applied to a metal surface,protect the metal during heat treatment and have the advantage ofspalling from the metal without the necessity of complicated, expensive,or damaging after-treatments.

In a particular embodiment of this invention, the composition may beprovided by mixing an aluminum silicate, such as an alkalimetal-aluminum silicate (e.g., petalite) .or an alkaline earth aluminum(e.g., calcium silicate aluminum silicate), in conjunction with a leadand boroncontaining fiuxing constituent which functions as an int-er.-stitial filler material for the silicate component. In this embodimentof the invention, the relative proportions of silicon, aluminum, boron,and lead constituents are as stated above.

It is often desirable to providethe aluminum and sili con constituentsof the composition as a single mineral or compound which is anoxygenated compound containing aluminum and silicon, ordinarily referredto as a silicate. The silicon constituent may be supplied as a silicatewhich contains no aluminum. The particular silicate which is utilized inthis composition is not of any critical significance since any silicatefalling within the indicated group will work satisfactorily. It is,therefore, not essential that the silicate be perfectly pure, althoughthe purer the silicate the better the results. Exemplary of thesilicates which may be used are aluminum silicate, which is thepreferred material, magnesium silicate, calcium silicate, and bariumsilicate. These, as well as other usable silicates, form the majorconstituents of such minerals as kaolin, feldspar, talc, bentonite, ballclay and chain clay, all of'which can be used in this coatingcomposition. It is, however, preferred to use kaolin since it is thepurest form of aluminum silicate available in mineral form.

A coating composition which is constituted of a silicate selected fromthe above class will not function in the desired manner without theinclusion of a boron and leadcontaining fiux'ing constituent. In anembodiment of the invention this constituent is selected from the classconsisting of (a) lead borate and (12) lead oxide and boric acid. Thelatter two compounds produce lead borate upon heating and, therefore,function in the same manner as lead borate. While it is not intended tobe limited by the following explanation of the purpose of the bor-ateconstituent, it is believed that it acts as an interstitial fillermaterial for the silicate and, therefore, provides a coating which isalmost completely impervious to the passage of oxygen at the hightemperatures to which the coated article will be subjected during heattreatment. The borate constituent also serves to lower the meltingtemperature of the coating composition and additionally provides for alower initial temperature of protection. It thus increases the range ofprotection to from about 800 F. to about 2350 F.

The aluminum, silicon, boron, and lead constituents of the compositionmay be provided as minerals and compounds containing one or more of theconstituents in conjunction with oxygen, halogen, and an alkali oralkali earth metal. Thus, the composition of this invention may beprepared from compounds and minerals selected but not limited to thefollowing: Colemanite, Ca B O 'SI-I O; cyanite, Al SiO danburite, CaO-BO '2SiO datolite, Ca(BOH) SiO ganomalite, 4Ca O6PbO 6SiO H O; harmotome,(K BmO-Al Og SSiO; SE 0: hydromephelite, HNa Al Si O -3H O;kaliophylite, KAlSiOg kaolin, A1 0 2SiO 2H O; marialite,

matlochite, PbO-PbCl mendipite, 2PbO-lbCl orthoclase, K O-Al O '6SiOpetalite, Li O-Al O 8SiO quartz, Slo sassolite, B(OH) sodalite,

uanalcnaiusioin wollastonite, CaSiO mullite, Al Si O calcium aluminumsilicate, CaO-Al O -2SiO albite,

N320 A1203 neplrelite, NaAlSiO potassium aluminum silicate,

4 potassium aluminum borate, K(AlO) (BO amalcite, N21 O-Al O -4SiO -2HO; andalusite, Al SiO andesine, (CaO, Na O)Al O -4Si() amorthite,

CaCO-Al O -2Si0 amorthoclase, (Na, K) O-Al O -6SiO apophyllite, KO-8CaO-16SiO-l6H O; aXll'lite, nca Al ssno barysilite, Pb Si O bauxite,Al O -2H O; borax,

chabazite, CaAl Si O -8H O; pyroboric acid, H B O arthoboric acid, Hmetaboric acid, B (OH) boric anhydride, B 0 lead metaborate, Pb (B0 1-10; various lead oxides; lead meta silicate, PbSiO and the like.

The binding composition employed can be any organic polymeric, resinous,or plastic material which will serve to bind the particles of the otherconstituents together so as to form a continuous film on the surface ofthe metal to which it is applied. Hence, the binding composition can becomposed of acrylate resins, polymeric epoxy resins, polyurethanes,alkyd resins, copolymers of various resins, or other similar materials.The preparation of these resins is well known to those skilled in theart and can be found in such text books as Organic Chemistry, by Fieser& Fieser, published by D. C. Heath & Co., Boston. Non-limiting examplesof acrylate resins which can be used in the coating aremethylmethacrylate, ethylmethacrylate, n-propylmethacrylate,diethyleneglycolmethacrylate, and methylethacrylate. An example of analkyd resin is the condensation product of glycerol and phthalicanhydride. Other alkyd resins are well known to those skilled in theart. An example of an epoxy resin is a glycidyl polyether resin obtainedby the reaction of epichlorohydrin with 2,2-bis(4-hydroxyphenyl)propane.This produces a polymer in which the molecular units have terminal epoxygroups. Other epoxy resins prepared by the reaction of epichlorohydrinwith polyhydric alcohol such as 1,2,3-trihydroxypropane are well knownin the art. An example of a polyurethan is a diisocyanate of propyleneglycol of the general formula C II CNO, wherein n is a number taken fromthe series 0, 1, 2, 3, and having an average molecular weight ofsubstantially 2500. The diluent used in the preparation of thisinvention may be any of the'well known diluents employed with resins andpolymers in the paint industry. These include lower aliphatic ketones,lower alkyl esters of lower aliphatic acids, benzene, and lower alkylsubstituted benzenes, all containing up to about 14 carbon atoms.Nonlimiting examples of these diluents are acetone, methylethyl ketone,diethyl ketone, diisopropyl ketone, octyl pentyl ketone, methylacetate,-butyl acetate, octyl acetate, methyl propionate, octylhexanoate, benzene, toluene, xylene, ethyl benzene andtert.-butyl-benzene. In addition, water may be used as the diluent wherethe binder is compatible therewith, such as methyl cellulose.

The amount of binder employed in the composition of this invention mayvary from about 1 to about 5 percent, based on the total of inorganicsolids employed. The exact amount of binder employed depends upon thedegree of adherence necessary to maintain the coating on the metalsurface during heat treatment. Since the organic binder employeddecomposes at the heat treatment temperature, it is preferred to employonly the amount of binder absolutely required. Excess binder isundesirable because, upon decomposition, it may tend to form carbides ofthe metals in the particular alloy being treated. A preferred ratio ofbinder to inorganic solids in the composition of this invention is 2 toabout 4 percent.

The amount of binder employed may also be determined as a ratio of thesilicate constituent of the coating composition. The ratio of thesilicate constituent to resin in the coating composition can vary fromabout 10 to l to about 2 to 1. At least 10 weight percent of resin basedon the total weight of the silicate constituent is required in order toimpart sufficient adhesive quality to the coat ing material to permit itto stick to the metal and thus prevent oxygen from coming in contactwith the surface of the metal during the period when the coated metalarticle is being brought up to the temperature at which the organicconstituents of the coating are burned off in the furnace at thebeginning of the heat treatment cycle. On the other hand, the amount ofresin should be no more than about half the amount of silicateconstituent since coatings which contain resin in excess of this amounttend to form blisters during the burning off of the organic material atthe beginning of the heat treatment cycle. It is preferred to use 12parts of resin for every 100 parts of silicate.

The resin may contain various customary plasticizers, the nature andamount of which are well known to those skilled in the art and will notbe here discussed.

The amount of diluent employed with the coating can vary from 1 to timesthe amount of solids present in parts by weight. The amount of diluentcan be varied to conform to the desired manner of application, as forexample, brushing, spraying, or dipping. -A ratio of solids to diluentof 1 to 1 is found to be satisfactory when the coating is spread on thesurface to be protected with a spatula. For spray application, it isfound that the composition is of the proper consistency when the ratioof solids to diluent is about 1 to 10. Still greater amounts of diluentmay be employed if desired. Ordinarily however, amounts in excess ofthat which would give a ratio of solids to diluent of 1 to 10 give noadditional advantage and only increase the amount of diluent that mustbe evaporated from the coating.

An essential constituent of the composition of this invention, when itis desired to produce a coating which will readily spall from thesurface of stainless steels, copper and copper alloys, is a metalchloride selected from the class consisting of calcium chloride andbarium chloride. This constituent modifies the thermal coefiicient ofexpansion of the coating to alter it substantially from that of theabove noted alloys. When calcium or barium chloride is incorporated inthis composition, the coating completely shatters and drops from theheat treated surfaces during its cooling cycle to leave a bright metalfinish which requires no clean-up operations.

Nevertheless, inclusion of the calcium or barium chloride constituentwill not cause the coating to spall completely from the heat treatedsurfaces of any of the superalloys. While the coating performs itsprotective function better if calcium or barium chloride isincorporated, it is not essential to add one of them when the coating isintended for use with the superalloys. This is also true insofar asstainless steel and the other alloys are concerned since, in manyinstances, it may be desired to retain the coating on the metal surfaceduring a heat treatment which includes a multiplicity of heating andcooling cycles. For example, a commonly used heat treatment for astainless steel such as PH15-7Mo, requires heating the metal to 1900 F.for 30 minutes, followed by cooling to room temperature, heating to 1750F. for 10 minutes, cooling to '-100 F. for 8 hours, reheating to 1075 F.for 60 minutes and finally cooling to room temperature. If the coatingis one which will spall upon cooling, it will be necessary to recoat thearticle between heating cycles. There are, on the other hand, manysituations in metal treatment where a single heating cycle is employed,as where a full anneal is accomplished between metal form ingoperations. For these reasons, the coating composition of this inventionencompasses two specific embodiments, one containing calcium or bariumchloride and one excluding these components. I

Where the coating of this invention is used to protect the superalloysduring heat treatment, a simple dip for 10 minutes or less in an acidpickling bath, such as one consisting of approximately percent nitricacid and 5 percent hydrofluoric, will accomplish coating removal withoutdifliculty. It should be remembered that no coating compositions, priorto the instant one, have been developed which can be removed from thesuperalloys without use of abrasive means. Furthermore, suchconventional coatings fail in their protective function, whereas thecoating 'of this invention affords sufi'iciently complete protection topractically eliminate oxide scale formation.

In addition to a particular coating composition and a process for heattreating metals, this invention also provides coated metal articles inwhich the coating protects the surface from oxidation at ambientatmospheric conditions as well as at elevated temperatures. In thismanner, metal articles which are to be heat treated may be protectedfrom oxidation for long periods of time prior to the actual treatment ofthe articles at elevated temperatures.

A coating composition of this invention comprises 100 parts ofanaluminum silicate; from 50 to 400, and preferably 200, parts of a leadand boron-containing constituent such as (a) lead borate, (b) a mixtureof lead oxide and boric acid, and (c) a lead borosilicate glass frit;from 1 to 50, and preferably 12, parts of a binder; and from 0 to 50,and preferably 20, parts of a fluxing agent selected from the classconsisting of calcium chloride and barium chloride, all by Weight of'the total solids. It will also be found necessary to incorporatesufiicient diluent to provide spreadable consistency to the composition.A preferred composition is one containing 13 percent kaolin, 25 percentof a mixture of percent lead oxide and 15 percent boric acid, 1.5percent of acryloid resin, and .60 percent toluene. A particularlypreferred embodiment of the invention employs a lead borosilicate fritcontaining 60-65 percent lead as lead oxide, 20-25 percent boron as B 0and 10-15 percent silicon as SiO The general process for the preparationof the coating employed in this invention consists of incorporating theconstituents in a ball mill and grinding for a sufficient period of timeto reduce the particle size of the solids to less than approximatelymesh as measured by the Taylor standard sieve. It is preferred to employa ball mill lined with ceramic material and having ceramic tumblingballs therein. After removal from the mill, the composition may bediluted with additional diluent to whatever consistency is desired. Themetal article which is to be heat treated is then coated with thiscoating composition by spreading, brushing or spraying so as to providea coat having a thickness of from about 0.25 to about 5 mils andpreferably from 0.25 to 0.5 mil. The coating is then allowed to air-dryfrom about one to about 10 minutes in order to allow the excess diluentto evaporate. The coating is sufficiently dry when it is firm to thetouch and exhibits no tackiness. The coated metal article is then readyfor heat treatment. If the metal article is to be subjected to a singleheating cycle for purposes of giving it a full anneal, it may be placedin a furnace at ambient temperature and then heated to bring the coatedarticle first up to a temperature at which the organic material in thecoating is decomposed and burned off, leaving a film having no carbon orhydrogen therein. This usually occurs at from about 400 F. to 600 F.Following this, more heat is ap plied to bring the article to theparticular heat treatment temperature desired for the metal concerned.After the heat treatment, the coated metal article is air-cooled untilit is brought to ambient temperature. Upon cooling to room temperature,the coating will begin to peel and drop from the surface of thestainless steel specimen at about 500 F. Spelling will continue until nocoating remains adhered to the surface and a bright metal finish isrevealed. On the other hand, when an article composed of the same metalis subjected to the same heat treatment without the coating of thisinvention, the amount of scale formed is extremely thick and heavy andvery difiicult to remove. When knownco-atings are used, the scale whichis, nevertheless, formed is of less thickness but is still diificult toremove. With most such com positions, the scale can only be removed bymechanical means or by pickling in aqueous mineral acid solutions suchas those consisting of hydrofluoric and nitric acids for extendedperiods of time.

The following examples will more clearly illustrate the composition,process, and articles of this invention.

Example I A coating composition was prepared by mixing together 150parts of kaolin with 255 parts of lead oxide, 45 parts.

of boric acid, 30 parts of calcium chloride, 18 parts of acrylic resinsand 500 parts of toluene, milling this mixture in a porcelain ball millfor a period of about 48 hours, removing the composition from the mill,washing the mill out with 250 additional parts of toluene and adding themill wash to the final composition. This composition was then sprayedonto an Inconel X article and the excess toluene permitted to evaporateat room temperature, leaving a film 0.25 mil thick. Inconel X is atrademark for a super-alloy containing from 70 to about 77 weightpercent nickel, 14 to 16 percent chromium, 2.25 percent titanium, 0.7percent cobalt, 0.4 percent aluminum, 5 percent iron, 0.3 percentmanganese, with minor amounts of silicon, copper, carbon and sulphurmaking up the balance. The coated Inconel X article was then placed inan electric furnace which was at ambient temperature and heat was slowlyapplied, raising the temperature of the metal article to thedecomposition point of the coating. Prior to reaching a temperature ofabout 700 F., the carbonaceous materials had been oxidized and drivenoff, leaving a fused anhydrous silicate film on the surface of the metalas shown by subsequent spectral analysis. The application of heat wascontinued, raising the temperature to 1625 F. The furnace, containingthe Inconel X article, was maintained at this temperature for 4 hours.The heat treated article was then removed from the furnace and cooled inair to ambient temperature. As the specimen cooled, excess coatingspalled. Analysis of the remaining surface film showed that it comprisesa fused coating containing aluminum silicate, lead borate and calciumchloride. Although the coating adhered to the surfaces and wascontinuous in nature, photor'nicrographs indicated the absence of anyintergranular penetration of the Inconel X surfaces. The part was thenheat aged .by being placed back in the furnace at a temperature of 1300F. and maintained at that temperature for 10 hours, and then aircooled.Upon removal from the furnace the article was immersed in an aqueousacid solution containing 5 weight percent hydrofluoric acid and 20weight percent nitric acid for a period of 10 minutes. This removed theremaining coating, leaving a shiny smooth surface.

Example II The procedure of Example I was repeated using a film 0.5 milthick with the modification that the coated Inconel X article was placedin a furnace which Was maintained at about 1625 F. Thehydrocarbonportion of the coating immediately burned volf leaving afused aluminum silicate-lead borate-calcium chloride film on thesurface. The sample was maintained in the furnace for a period of 4hours, which was sufficient to relieve all stress within the metal.Thereafter, the article was removed and cooled as in Example I. The filmwas found to .be 0.5 mil thick and upon immersion in the acid solutionof Example I for a period of 15 minutes the scale was removed, leaving ashiny smooth surface.

Example 111 The procedure of Example II was repeated using a film 5 milsthick with the modification that the furnace was maintained at atemperature of 1925 F. for full anneal purposes. quent cooling, excessfilm spalled and the remainder of the fused coating was found to besubstantially 0.5 mil thick and was readily removed upon immersion inthe acid solution.

Example IV A coating composition was prepared as in Example I employing50 parts of china clay, parts of lead borate, 10 parts of bariumchloride and 3 parts, all by weight, of an alkyd resin obtained byreacting glycerol with phthalic anhydride. The coating compositioncontained one part of methylethyl ketone diluent to one part of thesolids. The composition was applied with the aid of a putty knife to thesurface of a H-ll type steel sheet measuring /8" x 6 x 24" so as toprovide a film substantially 2 mils thick. The general composition ofthe H-ll type steels, in percent by weight, is approximately 0.40percent C, 5 percent Cr, 1.3 percent M0, 0.5 percent V plus smallamounts of Mn, Si, and other alloying elements, with the balance beingiron. The coated piece of steel was allowed to stand exposed to theatmosphere for a period of six months without any indication ofdeterioration of the film or of corrosion of the steel beneath the film.The sheet was next placed in a furnace and maintained at a temperatureof 1850 F. for a period of onehalf hour. The sheet was cooled to ambienttemperature and as cooling progressed below about 500 F. or 600 F.,spalling of the coating became more and more apparent until a surface,containing no coating, remained at room temperature. No coating removalwas required.

A steel sheet was heat treated as in Example IV, but Without aprotective coating. An oxide scale formed on the surface which requiredabrasive cleaning for removal.

Example V The procedure of Example I was repeated in the preparation ofa coating composition consisting of 150 parts of feldspar, 200 parts oflead borate, 30 parts of calcium chloride and 25 parts of an epoxy resinobtained from the reaction of epichlorohydrin with2,Z-bis(4-hydroxyphenyl)propane in equimolar quantities. The ratio ofn-heptane diluent to combined resin and solids was substantially 10to 1. This composition was sprayed onto a Ren 41 specimen containing 51weightpercent Ni, 11 weight percent Co, 10 weight percent Mo, 19 weightpercent Cr, and 3 weight percent Ti to provide a film substantially 0.5mil in thickness. The film was allowed to air-dry about 5 minutes andthen the coated specimen was placed in a furnace maintained at 1950 R,where it was kept for 30 minutes. Following this, the specimen wasair-cooled and the coating on the heat treated specimen was found to besubstantially 0.5 mil in thickness. This scale was removed from thesurface by immersion in the acid solution described in Example I.

Heat treatment of uncoated Ren 41 at 1950 F. for one-half hour as inExample V produced oxide scale which could not be removed by immersionin the acid solution but had to be removed by abrasive methods.

Example VI A high carbon tool steel rod measuring 1 inch in diameter and3 inches in length was coated with the composition described in ExampleI to a depth of 1 mil. The analysis of this tool steel was 0.72 percentC, 0.25 percent Mn, 0.20 percent Si, 40 percent Cr, 18.25 percent W, 1.5percent V, with the balance Fe. The coated rod was then placed in afurnace maintained at a temperature of 1550 F., Where it was soaked for30 minutes. The specimen was then removed from the furnace and placed ina 2350 F. furnace where it was retained until it reached furnacetemperature. It was then quenched in oil, and water-washed to removeoil. The specimen had a bright, shiny metal finish. It was later coatedwith a composition identical to that of Example After full anneal for 30minutes and subse- I except that no calcium chloride was included. Afterthe diluent evaporated from the coating, the specimen was placed in afurnace maintained at a temperature of 1050 F. and retained therein fora period of two hours, after which it was air-cooled to roomtemperature. This same 1050 F. cycle was repeated in order to give thespecimen the desired temper. After air-cooling was completed, thespecimen was immersed in a conditioning bath, prepared in accordancewith Patent No. 2,861,015, and containing 50 percent sodium hydroxideand 1 percent sodium dichromate and retained therein for 5 minutes.Visual and micrographic examination of the specimen showed the surfaceto be bright and shiny and indicated a complete absence of anydecarburization or other surface degenerated conditions. The Rockwell Chardness of the specimen was 64.

Example VII The procedure of ExampleVI was repeated with an identicalspecimen but without application of any coating. During the heattreatment to 2350 F., excessive scaling and carburization to a depth of.020 inch per surface occurred.

Examples VI and VII illustrate the substantial advantages obtained inemploying the coating of this invention to protect a metal during heattreatment. While, in Example VII, a scale which was /2" thick formed asthe result of heat treatment, no scale formed on the specimen pre-coatedwith the composition of this invention, nor did any carburization occur.

Example VIII Following the procedure of Example I, a coating compositionof this invention was prepared by milling 300 parts of glass ceramicfrit containing 60-65 percent lead oxide, -15 percent silica, andpercent boric an-' hydride, with 150.5 parts kaolin, parts calciumchloride, and 600 parts of toluene. After milling, the mill was washedwith additional toluene and the toluene wash was combined with make-uptoluene to add an additional 100 parts of toluene to the composition.Prior to adding the wash and make-up toluene, 18.3 parts of acrylicbinder were dissolved therein. In this way, a portion of the siliconconstituent was added to the mixture along with the boron and leadconstituents and the majority of the silicon constituent was added tothe composition along with the aluminum constituent.

An alternate method for preparing compositions of this invention is tocombine the minerals or compounds used to provide the variousconstituents and form therefrom a frit by firing the mixture to anappropriate temperature. This combined frit is then milled and thediluent and binder added after milling. The relative proportions ofboron constituent, lead constituent, silicon constituent, and aluminumconstituent employed in the composition of this invention vary dependingupon the melting point desired for the final composition. The meltingpoint desired depends upon the nature of the heat treatment to be used.When a high melting point is required, the composition will be higher inthe aluminum and silicon conjstituents, while for a low meltingcomposition large amounts of the lead and boron constituents areemployed.

Although this invention has been fully described and illustrated, itwill be understood that this disclosure is for purpose ofexemplification and is not to be taken by way of limitation, the spiritand scope of the invention being limited only by the terms of theappended claims.

I claim:

1. A metal preheat-treat coating composition consisting essentially ofabout 12 percent kaolin; 24 percent of a lead borosilicate fritcontaining 60-65 percent lead oxide, 10-15 percent silicon dioxide and2025 percent boric an- 10 hydride; 2.5 percent calcium chloride; 1.5percent acryloid resin; and 60 percent toluene.

2. A process for heat treating a stainless steel article comprisingapplying a coating to the surfaces thereof, the coating compositionconsisting essentially of oxygenated inorganic nonmetallic constituentsof aluminum, silicon, boron and lead, containing 10-40 percent aluminumconstituent, based on aluminum oxide, 10-40 percent silicon constituent,based on silicon dioxide, 20-70 percent lead constituent, based on leadoxide, and v15-25 percent boron constituent, based on boric anhydride;heating the coated article to fuse said coating; thereafter heattreating said coated stainless steel article at a predeterminedtemperature, and subsequently cooling said coated stainless steelarticle to room temperature.

3. A process for heat treating an article constructed of a superalloycomprising applying a coating to the surfaces of said article, thecoating composition consisting essentially of parts of aluminumsilicate, from 50 to 400 parts of a mixture of lead oxide and boricacid, from 1 to 50 parts of a polymeric organic binder, and sufiicientdiluent to give the composition a spreadable consistency; heating saidcoated article to a temperature sufficient to fuse said coating;thereafter heat treating said coated metallic article at a predeterminedtemperature; subsequently cooling said article to room temperature,thereby causing spalling of excess fused coating, and finally immersingsaid article in an acid solution to completely remove any remainingtraces of coating.

4. A process for heat treating a stainless steel article, the processcomprising applying a coating composition to the surfaces of saidarticle; heating the coated article to fuse the coating; thereafter heattreating the coated article to a predetermined temperature, andsubsequently cooling the coated article to room temperature; saidcoating composition consisting essentially of 100 parts of an aluminumsilicate; from 50 to 400 parts of a lead and boron containingconstituent selected from the class consisting of (a) lead borate and(b) a mixture of lead oxide and boric acid; up to about 50 parts ofcalcium chloride as a fluxing agent; from 1 to 50 parts of a polymericorganic binder, and sufiicient diluent to give the composition aspreadable consistency.

5. A process for heat treating a stainless steel article, the processthe comprising applying a coating composition to the surfaces of saidarticle; heating the coated article to fuse the coating; thereafter heattreating the coated article to a predetermined temperature, andsubsequently cooling the coated article to room temperature therebycausing the coating to spall completely from said surfaces and to leavesaid surfaces scale-free and non-carburized; said coating compositionconsisting essentially of about 12 percent kaolin; 24 percent of a leadborosilicate frit containing 6065 percent lead oxide, 10-15 percentsilicon dioxide and 20-25 percent boric anhydride; 2.5 percent calciumchloride; 1.5 percent acryloid resin; and .60 percent toluene.

References Cited by the Examiner UNITED STATES PATENTS 1,764,609 6/ 30Dean 14822 2,121,606 6/38 McCulloch 14822 2,485,176 10/49 Waterfall14822 2,889,238 6/59 Long et a1 148--27 X 2,906,907 9/59 Peras 10649 XFOREIGN PATENTS 552,663 4/43 Great Britain.

DAVID L. RECK, Primary Examiner.

MARCUS U. LYONS, WINSTON A. DOUGLAS,

Examiners.

5. A PROCESS FOR HEAT TREATING A STAINLESS STEEL ARTICLE, THE PROCESSTHE COMPRISING APPLYING A COATING COMPOSITION TO THE SURFACES OF SAIDARTICEL; HEATIG THE COATED ARTICLE TO FUSE THE COATING; THEREAFTER HEATTREATING THE COATED ARTICLE TO A PREDETERMINED TEMPERATURE, ANDSUBSEQUENTLY COOLING THE COATED ARTICLE TO ROOM TEMPERATURE THEREBYCAUSING THE COATING COMPOSITION CONSISTING ESSENTIALLY OF ABOUT 12PERCENT KAOLIN; 24 PERCENT OF A LEAD BOROSILICATE FRIT CONTAINING 63-65PERCENT LEAD OXIDE, 10-15 PERCENT SILICON DIOXIDE AND 20-25 PERCENTBORIC ANHYDRIDE; 2.5 PERCENT CALCIUM CHLORIDE; 1.5 PERCENT ACRYLOIDRESIN; AND 60 PERCENT TOLUENE.